1. Field of the Invention
The invention relates to discarding sabot projectiles, and more specifically to sub-caliber fin-stabilized armor penetrating projectiles, which contain therein rod penetrator cores, and an integral tracer of suitable pyrotechnic composition.
2. Description of the Prior Art
Heretofore, two types of armor piercing projectiles were utilized in small caliber gun systems. One of the designs was of a conventional projectile shape and was full-bore diameter, consisting of a combination of high strength steel or high density material as a penetrator swaged or inserted into a suitable jacket or sleeve material. At the projectile base was an opening for a tracer cavity of adequate depth and diameter to provide a clear visual trace of the entire projectile trajectory. This type of full-bore projectile utilized density of high strength penetrator and to some extent the jacket or sleeve material and its geometry to affect armor penetration. This type of projectile had severely limited armor penetration capability at target engagement ranges beyond several hundred meters, due to its high drag configuration.
It has been demonstrated that sub-caliber high density rod type penetrators are capable of penetrating significantly more armor than the full-bore projectiles at target ranges beyond several hundred meters. This is due to the high density rod's more efficient armor penetration geometry and the greater mass per cross sectional area of the sub-caliber rod flight projectile, which results in it losing less velocity from aerodynamic drag. To take advantage of the rod's high ballistic coefficient and to provide increased initial launch velocities, sabots were designed to encapsulate the rod penetrator during handling, storage, and gun firing, and to discard shortly after exiting the muzzle, thus allowing only the rod penetrator to continue in flight toward the target. One type of discarding sabot projectile has been demonstrated in small caliber guns to provide increased armor penetration over full-bore projectiles. This was the Armor Piercing Discarding Sabot (APDS) projectile, which utilized a spin stabilized sub-caliber penetrating core as the flight projectile. APDS projectiles using high density rod penetrators have been developed for guns from caliber 5.56 millimeter through caliber 120 millimeter. Given aerodynamic considerations, APDS projectile designs below caliber 25 millimeter did not allow the inclusion of a tracer cavity without degrading penetrator performance. The tracer cavity in these projectiles significantly reduces the available high density rod material required for armor penetration.
It has been demonstrated that armor piercing fin stabilized discarding sabot (APFSDS) projectiles penetrate more armor at greater ranges than spin stabilized APDS projectiles, due to the longer allowable penetrator lengths that can be launched and flown to the target with accuracy and stability. APFSDS projectiles utilizing high density sub-caliber rod penetrators have been developed for both rifled barrel and smooth bore guns from caliber 25 millimeter through 140 millimeter, and these designs have permitted the incorporation of an adequate tracer cavity in the rear of the flight projectile without degradation of the rod's armor penetration performance. Flechette type APFSDS projectiles utilizing high strength or high density rod penetrators have been developed for small caliber 5.56 and 7.62 millimeter rifle systems, but without allowance for a tracer cavity in the flight projectile.
Fin stabilized APFSDS projectile designs incorporating an adequate tracer cavity and developed for larger caliber systems did not efficiently scale down to small caliber projectiles due to the complexity of their sabot geometries which were optimized for the unique parameters of the larger caliber systems. Prior fin stabilized APFSDS projectile designs for smaller caliber 5.56 and 7.62 millimeter guns did not provide for a tracer cavity in the rear of the flight projectile.
Defining tracer adequacy involves consideration of complex projectile, gun, and pyrotechnic parameters. Fundamentally, however, tracer burn time, and therefore tracer cavity depth requirements are based on the time of flight of the projectile to maximum effective range. The longer the flight time is; the deeper the tracer cavity has to be. Tracer cavity diameter requirements, however, are based on the maximum effective range of the projectile trajectory. A larger diameter tracer cavity is required for projectiles which must be observed at a greater effective range. One tracer cavity tradeoff which then becomes apparent is that a slower projectile, fired to the same effective range as a faster projectile, requires a deeper tracer cavity. However, the intent of designing a faster projectile is generally to achieve greater effective ranges. Therefore, the faster projectile, although having the same time of flight, and, therefore, the same tracer cavity depth as the slower projectile, travels to a greater range in that same amount of time so that it requires a tracer cavity of larger diameter to be adequate for visual tracking.
The performance trends, therefore, in projectile design which affect tracer adequacy can be generalized to efficiently designed full-bore, APDS, and APFSDS projectiles as follows. For the same gun system, an APFSDS projectile will have a greater effective range than an APDS projectile, which will have a greater effective range than a full-bore projectile. Therefore, for that gun system, the APFSDS projectile will require a larger diameter tracer cavity than its APDS counterpart, which requires a larger diameter tracer cavity that the full-bore projectile. The time of flight of each of these projectiles to maximum effective range, however, will largely remain the same, since the faster flying projectiles are flying correspondingly farther in the same amount of time. Therefore, tracer cavity depths will largely remain equal between the full-bore, APDS, and the APFSDS projectiles.
For typical projectiles in 12.7 millimeter (0.50 inch) caliber guns, tracer cavity depths of approximately 12 millimeters are required for adequate burn times for trajectories lasting approximately two seconds. This gives a full-bore projectile trajectory trace to approximately 1000 meters, an APDS trace to 1500 meters, and an APFSDS trace to 2000 meters. In 20 millimeter caliber guns, tracer depths must be doubled to approximately 24 millimeters for adequate trajectory observation beyond 3000 meters, or 4 seconds for an APDS projectile flight time. In general, approximately 6 millimeters of tracer cavity is required for every second of flight time.
Since APFSDS projectiles will fly farther than either full-bore or APDS projectiles from the same gun, APFSDS projectiles require a larger diameter tracer cavity to maintain visual detection at greater ranges. Ironically, the APFSDS projectiles, which are the most demanding on tracer diameter requirements, are the most difficult designs to incorporate an adequately large diameter tracer in small caliber systems, since they are based on flying a long but slender rod penetrator to a more distant target.
Tracer cavity diameter, based on tracer light intensity, scales proportional to the range of the projectile squared. For adequate trace detection up to an effective range of 3500 meters, a tracer cavity diameter of approximately 16 millimeters is required. This is typical of large caliber tank gun projectiles, such as for the 120 millimeter cannon. In a 120 millimeter diameter projectile, the tracer cavity diameter represents approximately 17% of the gun bore diameter. Tracer diameters of 8 millimeters are sufficient for an effective range of 2500 meters, and are typically found in projectiles for guns of 30 millimeter diameter. In a 30 millimeter projectile, the tracer cavity represents approximately 26% of the gun bore diameter. In small caliber gun systems, for example the 12.7 millimeter (0.50 inch) caliber heavy machinegun, a minimum tracer cavity diameter of approximately 5 millimeters is required for adequate trace to a maximum effective range of 2000 meters. This small caliber tracer diameter requirement represents nearly 40% of the small caliber gun bore diameter. One sees that when scaling projectile designs down from large caliber to small caliber, the requirements for tracer cavity diameter scales up significantly, from 13% of bore diameter to nearly 40% of bore diameter. Therefore, existing large caliber APFSDS projectile designs are inappropriate for use in small caliber systems.
Clearly, APFSDS projectiles, which include a tracer cavity in the rod penetrator, exist for gun systems with bore diameters greater than or equal to 30 millimeters. It is equally true that the proportions of the tracer cavity in these large projectiles is adequate for these gun systems. However, it is erroneous to conclude on the basis of these large caliber designs that an adequate tracer cavity can be easily incorporated into an APFSDS projectile for gun systems of caliber less than 25 millimeters, where the tracer cavity requirements are double the proportions required in the larger caliber projectiles. Incorporating an adequate tracer cavity in a small caliber APFSDS projectile requires significant design tradeoffs not required in larger caliber projectile systems, and the use of design methodologies heretofore unpracticed.
Therefore, prior APFSDS projectile designs which offer greater armor penetration were inappropriate for small caliber guns where a tracer cavity is required, and prior APDS projectile designs did not allow for an effective armor penetrator with a tracer cavity. Prior small caliber APDS projectile designs are also faulty in that the sabot discard process introduces trajectory inaccuracies for the rod projectile. Prior larger caliber APFSDS projectile designs, however, have demonstrated that proper sabot and obturator design can provide trajectory accuracies comparable with those of full-bore projectiles.
Accordingly, it is advantageous to provide an armor piercing fin stabilized discarding sabot (APFSDS) projectile for small caliber guns which minimizes sabot parasitic weight and structural complexity, facilitates rapid sabot separation upon muzzle exit without introducing trajectory inaccuracies for the rod projectile, maximizes armor penetrator weight and length, and provides for an adequate tracer cavity in the rear of the flight projectile.